This invention relates to fiber-resin composite pultrusion methods and products, and, more particularly, to a method for pultruding rods of alternately hollow and solid cross section.
The basic techniques for running filaments through a resin bath and then into a long heated die tube to produce a cured composite of the same shape as the die tube has been known for some time. See, for example, U.S. Pat. Nos. 2,948,649 and 3,556,888. The drawback of this method is that, for certain applications, the solid extruded product is unacceptably heavy. One such application is a composite replacement for wooden handles, such as a shovel.
The weight problem could be alleviated by means of an existing process to extrude hollow tubes utilizing a die tube with the center filled, leaving an annular cross section through which the fibers are pulled. Although this process can produce a light, hollow tube, it would clearly have a bending or crushing strength significantly lower than that of a solid rod, and therefore would not be suitable for certain high-stress applications, such as shovel handles. Moreover, conventional machines for producing continuous hollow tubing are extremely expensive.
The bending stress problem can be solved by producing a rod which is hollow throughout most of its length but is solid in the area of maximum stress, near the head of a shovel, for example. The current pultrusion techniques provide no convenient and inexpensive way to produce such a reinforced product. However, in accordance with presently known practices, the reinforced tubing referred to above could be produced in several ways. For example, a solid pultruded rod could be bored out through most of its length, or a hollow pultruded tube could have a solid plug glued into its center for some fraction of its length. The boring method has the disadvantage of wasting material and being time consuming. The plugging method has the additional disadvantage of lowered strength due to the imperfect fit of the plug. Both of these methods would be too costly to provide an economical means of mass producing shovel handles or the like.
Accordingly, there has existed a need for a convenient and effective method for producing a continuous rod with an alternately solid reinforcing and hollow cross section in a quick and inexpensive manner. As will become apparent from the following, the present invention satisfies this need and provides other related advantages.